Results: The mean prostate CTV volume was 74.7±22.8 cc (range: 24.8-113.7 cc). Initial treatment plans provided target D90 = 107.7±2.0% (103.0-111.2%) and V100 = 98.5±0.8% (97.1-99.5%). The D0.1cc to OARs was 77.3±2.3% (rectum), 84.2±2.0% (bladder), and 106.8±1.6% (urethra). The mean target coverage was not significantly different for 1-2 mm shifts. Shifts between 3 and 6 mm still met most dosimetry objectives, but provided progressively less good target coverage and doses to OARs. Displacements of greater than or equal to 7 mm resulted in dosimetry that did not meet our dosimetry objectives.

Conclusion: Small changes in catheter position result in significant alterations in dosimetry and treatment delivery. The target coverage (D90 and V100) and bladder dose are reduced and the rectal and urethra doses are higher in direct proportion to the degree of displacement. It is essential to check and correct interstitial catheter positions before each HDR fraction to accurately deliver the planned radiation dose.